1. Field of the Invention
The present invention relates, in general, to the communication arts and, more particularly, to optical fiber communication systems in which multiple information channels are carried on a single mode optical fiber.
2. Description of the Prior Art
As optical fiber technology has advanced, there has been a general appreciation and recognition that optical fibers can be fabricated having the capacity to transfer information at extremely high rates. To exploit the maximum information transfer rates possible with optical fiber systems, however, the data must either be time-division multiplexed or multiple, wavelength-distinct information channels must share a common communications path. With respect to time-division multiplexing, there is, apparently, no electronic circuitry available that is capable of switching at pulse rates which approach the information handling rates of single mode optical fibers (viz., 5.times.10.sup.9 pps). Additionally, there previously has apparently been little progress with the latter wavelength division multiplexing approach because practical solutions to its problems in optical fiber systems have not been generally known. These problems in general are related to the difficulty in obtaining low cost, spectrally narrow, stable sources; the difficulty, particularly with single mode fibers, in splicing fibers one to another and in coupling energy from sources to fibers; the difficulties associated with amplifying signals; and the difficulty in selectively adding and removing signals to and from fibers.
With respect to spectrally narrow, stable sources, solid-state laser diodes, which are a preferred source, are comparatively expensive. Consequently, their utilization in an optical fiber communication system is only economically justifiable if high bit rates are attainable. For example, if a light-emitting diode and a graded index optical fiber can transfer 5.times.10.sup.7 pulses per second (pps) over a 20 kilometer (km) trunk line, and, if a laser diode and a single mode optical fiber can transfer 5.times.10.sup.9 pulses per second over the same length optical fiber, the latter laser diode and single mode fiber combination represents a cost factor 100 times greater than that for the former combination since laser diodes that have an acceptable operating lifetime presently cost more than 100 times as much as a light-emitting diode.
However, recent advances have been disclosed in the coupling area. For example, in U.S. Pat. No. 3,342,499, filed Mar. 31, 1980 for COMMUNICATIONS TUNING CONSTRUCTION, and U.S. patent application Ser. No. 331,052, filed Dec. 16, 1982 for RESONANT CAVITY FILTERS, structures are disclosed for wavelength multi-plexing signals into single mode optical fibers. Such inventions, including the inventions of U.S. Pat. No. 4,315,666 for COUPLED COMMUNICATIONS FIBERS and the above-referenced inventions by the present inventor, have made it possible to carry many wavelength channels on one single mode optical fiber and to selectively introduce or remove any of these wavelength-distinct channels at any point on the communications line with minimal interference with the other channels and while maintaining communication line integrity. Thus, it is possible to attain high channel capacity in the construction of an optical fiber communication system which can carry a very large cumulative or aggregate information flow without using exceedingly high modulation rates on any one given channel.
As is well-known, practical optical fiber communication systems utilizing wavelength-distinct modulated channels must carry information over an appreciable distance. As is also known in the art, despite significant advances in diminishing the attenuation characteristics of optical fibers, signals transmitted through optical fibers are attenuated by the cumulative and combined effect of absorption and scattering. While the attenuation rates of existing optical fibers are quite low, signal diminishment with increasing transmission distance nonetheless requires periodic signal reconstitution or amplification. In the past and in accordance with classic communication system practice, periodic signal amplification has been achieved using in-line repeaters which are inserted into the communications line thus interrupting the integrity of the line. The classic repeaters include an input port for receiving the attenuated signal, an amplifier or other signal processing device, and an output port coupled to the downstream communications line and to which the reconstituted signal is applied. In these systems, the attenuated signal is presented to a photoelectric device which converts the optical signal into a corresponding electrical signal. An electronic amplifier or signal processer then amplifies and conditions the signal to compensate for signal deterioration. Thereafter, the electrical signal is presented to another photoelectric device which provides an optical signal which is then launched onto the downstream optical fiber. The conversion of the optical signal to an electrical signal and reconversion back to an optical signal imposes an artificial limitation on an otherwise entirely optical system. In addition, the classic in-line repeater approach presents an undesirably high system-wide sensitivity to single repeater failure, because failure of the repeater destroys the line integrity.
In addition to the above, it can be appreciated that the fabrication of a repeater for an optical fiber communications system carrying many wavelength-distinct channels can become quite complex and expensive, since each channel must be removed separately from the line, processed separately to effect reamplification and conditioning, and remultiplexed with the other signals onto the next downstream optical fiber segment.
With respect to purely optical amplification, the art has shown devices relying on stimulated Raman scattering to convert the wavelength of one source to an amplified output at another wavelength, but there apparently has been no suggestion utilizing Raman scattering for the amplification of information-bearing signals, particularly multiplexed signals.
Pursuant to the above, it is a broad overall object of the present invention to provide a simple, reliable, and relatively economical optical fiber communication system capable of transferring information at high data rates.
Another object of the present invention is to provide an optical fiber communication system in which transmitted signals are amplified by purely optical mechanisms rather than by the electronic techniques used in the past and by which information can be transmitted by wavelength modulation using a plurality of information carrying wavelength-distinct channels.
Another object is to provide a means by which signals carried on an optical fiber transmission line can be collectively amplified without interrupting the line.
Yet another object is to provide a redundant amplification means along an optical fiber transmission line carrying several channels.
And another object is to provide a system which includes means for automatically controlling amplifier power levels.
Still another object is to provide a spectrally narrow, low cost, stable optical signal generator capable of being modulated.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.